Mourachkine A. Room-Temperature Superconductivity

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284 ROOM-TEMPERATURE SUPERCONDUCTIVITY

logically disordered. This can be achieved in a few ways. As discussed above,

the hexagons in graphene can be replaced by pentagons and heptagons. Some

carbon atoms in graphene can be substituted by B, N or Al [67]. In the frame-

work of our project, B and Al are probably better than N because each N adds

an additional electron to graphene, while room-temperature superconductivity

requires holes. Instead of graphene sheets, one can for instance use nanos-

tripes of graphene [70]. Alternatively, one may use the molecules of hexaben-

zocoronene shown in Fig. 1.3c or other large molecules of conjugated hydro-

carbons. To stabilize structurally graphene nanostripes, hexabenzocoronene

or other large molecules of conjugated hydrocarbons, one can prepare a sin-

gle crystal consisting of two alternating layers: graphene and a layer of one

of these large molecules or graphene nanostripes. Then, these single crystals

must be doped by magnetic atoms/molecules responsible for long-range phase

coherence.

In principle, it should be not a problem for bisolitons to occur in graphite and

graphene-based compounds above room temperature. The main problem for

the occurrence of bulk superconductivity in graphite and other organic com-

pounds above room temperature is the question of the onset of long-range

phase coherence. One can dope graphite by atoms/molecules spins of which

are ordered antiferromagnetically after the diffusion. Alternatively, one may

try to enhance intrinsic weak ferromagnetism of graphite/graphene.

Since the bonds between adjacent layers in graphite are weak, an artiﬁcial

“sandwich” doping can be used for graphite. By using the electrochemical

method to dope organic compounds [63], one cannot control the positions of

dopant species in the crystal structure after the diffusion. Furthermore, the

doping occurs only in a thin surface layer. For graphite however, one can

consciously control the positions of intercalant species. The idea is as follows.

By using a scanning tunneling microscope (STM), one can manipulate single

atoms/molecules putting them into the proper positions on a clean graphite

surface [71]. After such a delicate doping, one can cover the dopant species

by one or two graphene sheets. The STM doping is then repeated again and so

on. In the future, this procedure can in principle be computerized.

Fullerenes. Closed-cage molecules consisting of only carbon atoms are

called fullerenes. A molecule of the fullerene C

is schematically shown in

Fig. 3.17. There are other fullerenes such as C

100

etc.

The alkali-doped fullerenes (fullerides) able to superconduct were discussed

in Chapter 3. The unit cell of superconducting fullerides M

is depicted

in Fig. 3.18, where M is an alkali atom. The superconducting fullerides

are electron-doped. To exhibit room-temperature superconductivity, the sin-

gle crystals of C

must be doped by holes. Thus, one should ﬁnd suitable

dopant species for this purpose.

Room-temperature superconductors 285

Theoretical calculations show that fullerenes having a diameter smaller than

that of C

, such as C

[72] and C

[73], are able to exhibit a higher value of

relative to that of C

. Hence, in addition to buckyballs C

, the fullerenes

and C

are also promising candidates with which to form a room-

temperature superconductor.

One can use fullerenes not only in pure but also in polymerized form. As

an example, Figure 10.6 shows various one- and two-dimensional polymeric

solids formed from C

. Similarly to graphite, polymerized rhombohedral C

displays weak ferromagnetism above room temperature [67–69].

In addition to experiments on single crystals of pure or polymerized fulle-

renes, the fullerenes can also be used in a combination with graphite or/and

nanotubes. One can intercalate the graphene sheets in graphite by fullerenes,

as sketched in Fig. 10.7. Such an intercalation can for example be achieved by

(b)

(c)

(a)

Figure 10.6. Various one- and two-dimensional polymeric solids formed from C

[74]. The

balls are shown schematically.

286 ROOM-TEMPERATURE SUPERCONDUCTIVITY

Figure 10.7. Intercalation of graphene sheets by C

molecules.

using the STM doping discussed above. In this case, the intercalation of these

graphene–fullerene compounds by “magnetic” species can simultaneously be

done by STM.

Unlike graphene and other long organic polymers, the fullerenes have an

advantage to be packed into any form. Using insulating or semiconducting

nanotubes, one can form a one-dimensional “wire” from fullerenes by pack-

ing them into the interior of one of these nanotubes, as schematically depicted

in Fig. 10.8a. Inorganic single-walled nanotubes which are either insulat-

ing or semiconducting have recently been reported in the literature, such as

MoS

[75], TiO

[76] and BN [77]. Theoretically, B

O and BeB

nanotubes

may exist as well [78]. In practice, the boron nitride nanotubes have already

been ﬁlled successfully by C

molecules, forming quasi-one-dimensional in-

sulating wires [77]. The utilization of BN nanotubes having various diame-

ters results in different stacking conﬁgurations of C

molecules [77]. In the

framework of our project, one should ﬁll the nanotubes not only with fullerene

molecules but also with “magnetic” species, sticking to a certain order in the

ﬁlling. This ﬁlling order should be a subject for a separate investigation. De-

pending on the diameter of a nanotube, one may use nanoscale pistons for ap-

plying a pressure, as sketched in Fig. 10.8b. By varying the pressure, one can

change the ﬁlling factor and, therefore, the T

value. These nanoscale pistons

in Fig. 10.8b can also be used as electrical contacts. Since carbon nanotubes

which will be discussed next can also be insulating or semiconducting, one

may use them instead of inorganic nanotubes.

Carbon nanotubes. In addition to spherical fullerenes, tubular carbon-

based structures are also called fullerenes. Here we shall call them carbon nan-

otubes or just nanotubes for short. Carbon nanotubes can be multi- and single-

Room-temperature superconductors 287

(b)

(a)

- magnetic species

Figure 10.8. (a) C

molecules inside an insulating nanotube [77], intercalated by magnetic

atoms/molecules. (b) Applying a pressure to C

molecules shown in plot (a). The nanoscale

pistons can be used as electrical contacts.

walled. For simplicity, we shall discuss the single-walled nanotubes. They

show metallic, insulating and semiconducting properties depending on the he-

licity with which a graphene sheet is wrapped to form the tubule. The armchair

nanotubes are usually metallic, while the zigzag ones are semiconducting. Fig-

ure 3.20 shows a piece of armchair nanotube. Similarly to graphite, the carbon

nanotubes may also exhibit weak ferromagnetism [67–69]. Two nanotubes can

be joined by electron beam welding, forming a molecular junction [79].

Due to their remarkable electronic and mechanical properties, a great future,

in the context of practical application, awaits the carbon nanotubes. The nan-

otubes are also a promising candidate with which to form a room-temperature

superconductor. As discussed in Chapter 3, the single-walled carbon nan-

otubes with a diameter of 4.2 ± 0.2 A

◦

exhibit bulk superconductivity below

 15 K [36]. The nanotubes with a smaller diameter may display a higher

. The onset of local superconductivity was observed in single-walled carbon

nanotubes containing a small amount of the magnetic impurities Ni and Co at

645 K [38]. By embedding these nanotubes into a dynamic magnetic medium,

one can witness bulk superconductivity above 450 K.

288 ROOM-TEMPERATURE SUPERCONDUCTIVITY

Since the electronic properties of the nanotubes depend on the wrapping

angle of a graphene sheet, one can invent a technique of twisting a nanotube

along its main axe. In such a way, one can vary the pairing temperature T

and,

as a result, T

. This suggestion is obviously for the distant future.

The carbon nanotubes can in principle be used in a combination with gra-

phene sheets. They will structurally support the nanotubes, as sketched in Fig.

10.9. The “magnetic” species can for example be situated between the nan-

otubes. Alternatively, instead of graphene sheets, one can use layers of mag-

netic materials ordered antiferromagnetically. The nanotubes in the adjacent

layers can for instance be oriented perpendicular to each other. This will make

the superconductor two-dimensional.

Figure 10.9. Intercalation of graphene sheets by carbon nanotubes. In the adjacent layers, the

main axes of nanotubes may be mutually orthogonal.

Single-walled carbon nanotubes ﬁlled with C

molecules are called

peapods. In peapods, the inner diameter of nanotubes is slightly larger than

the outer diameter of C

molecules. In order to discuss the following idea,

it is worth to recall that, in a solid, only dynamic spin ﬂuctuations are able to

mediate the long-range phase coherence. In the cuprates, for example, charge

ﬂuctuations in the CuO

planes induce these dynamic spin ﬂuctuations. By

analogy with the cuprates, C

or other fullerene molecules moving inside a

nanotube, as shown in Fig. 10.10a, can induce dynamic spin ﬂuctuations in the

magnetic surroundings. The more practical cases are depicted in Figs. 10.10b

Room-temperature superconductors 289

(b)

(c)

(a)

Figure 10.10. Moving C

molecules inside nanotubes: (a) a straight nanotube; (b) two par-

allel nanotubes closed at each end by rounded pieces of nanotubes, and (c) a round nanotube.

The purpose of such a dynamics is explained in the text.

and 10.10c: the fullerene molecules remain always inside the same nanotube.

However, in order to realize this idea, one must invent a technique allowing

to initiate the fullerene-molecule movement relative to a nanotube. Instead of

carbon nanotubes, one can in principle use the inorganic nanotubes discussed

above. Ideally, these inorganic nanotubes can be magnetic themselves; then,

there is no need for an additional doping procedure.

4.1.2 Living tissues

It is worth to recall that the main idea of the whole book is that, in order

to synthesize a room-temperature superconductor, we may use some of Na-

ture’s experience accumulated during billions of years, even if, this experience

in principle has nothing to do with superconductivity (see the discussions in

Chapters 1 and 8). After all, Nature is smarter than humans.

In principle, one can use living tissues to form a room-temperature su-

perconductor. All the descriptions presented in the previous subsection can

equally be applied to living tissues which are usually one- or two-dimensional.

One should use living tissues containing bisolitons. The main question is what

290 ROOM-TEMPERATURE SUPERCONDUCTIVITY

living tissues contain the electron pairs. From Chapter 1, we know that (i) in

redox reactions, electrons are transferred from one molecule to another in pairs

with opposite spins, and (ii) electron transport in the synthesis process of ATP

(adenosine triphosphate) molecules in conjugate membranes of mitochondria

and chloroplasts is realized by pairs.

Mitochondria and chloroplasts are integral parts of almost every living cell.

In principle, one can easily use their membranes to form a room-temperature

superconductor as described in the previous subsection. The redox reactions

occur practically in every cell. One should ﬁnd out what parts of the cells

are responsible for the redox reactions, and then use these tissues to form a

room-temperature superconductor.

In addition to these tissues, DNA (deoxyribonucleic acid) is also a good

choice: proximity-induced superconductivity was already observed in DNA

below 1 K [37]. It is also assumed that DNA will exhibit genuine supercon-

ductivity if one can ﬁnd a technique to dope it. The double helix of DNA

has a diameter of 20 A

◦

, and it can be a few microns long. So, DNA is truly

a one-dimensional system. Charge transport through DNA is crucial for its

biological functions such as the repair mechanism after radiation and biosyn-

thesis. So far, the ﬁndings concerning conductivity of DNA are controversial

[80]. Some measurements indicate that DNA behaves as a well conducting

one-dimensional molecular wire. In contrast, other measurements show that

DNA is insulating. The available data are more or less consistent with a sug-

gestion that DNA is a wide-bandgap semiconductor [80]. This proposal is also

in good agreement with the idea that electron transport in DNA occurs due to

electrosolitons and/or bisolitons, accompanied by molecular distortion (local

deformation of the lattice).

One of the main advantages of DNA is that it can easily be attached to

electrical contacts, as shown in Fig. 10.11. A large single molecule of double-

stranded DNA can be manipulated by attaching short, single-stranded DNA

molecules to each of its ends. Then, chemical labels on the ends of these

Receptor

Chemical

label

Single-stranded

synthetic DNA

DNA

Receptor

Golden

bead

Chemical

label

Figure 10.11. A single molecule of double-stranded DNA with single-stranded DNA

molecules attached to each of its ends [81]. Chemical labels on the ends of single-stranded

molecules are used to attach the DNA to an electrical lead or to a bead which can then be used

as an electrical contact. This conﬁguration is in fact used for manipulation of DNA molecules

[81], but can stand duty as an electrical circuit.

Room-temperature superconductors 291

single-stranded molecules are used to attach the DNA molecule to electrical

leads or microscopic golden beads, as depicted in Fig. 10.11.

It is worth noting that, in living cells, DNA molecules are in a solution,

and they actively interact with the solution. Therefore, one should consider

to experiment with DNA not only in a dry environment but also in the proper

solution.

The experiments should not be restricted by the living tissues discussed

above, one can use other tissues as well. The main requirement for them is

that they must contain bisolitons.

4.1.3 Oxides

Oxides, as a class of materials, deserve to be considered as materials able

potentially to superconduct at room temperature because:

at present, oxides are the largest group of superconductors having the high-

est growth rate;

at present, oxides exhibit the highest value of T

(cuprates);

at least, one oxide (Ag

) exhibits some signs of local superconduc-

tivity above room temperature [64], and

oxygen, as a chemical element, is a unique acceptor of electrons and plays

a crucial role in the living matter.

All the living matter cannot function without oxygen: the redox reactions

are essential part of biochemical processes occurring in living organisms, in

some of which electrons are transferred from one molecule to another in pairs

with opposite spins (see the previous subsection). Oxygen is also a constituent

element of DNA.

Contrary to the widely-used expression “the living matter is organic” (in-

cluding humans), oxygen, in fact, represents the heaviest part of a human body

(61 %), while C and N contribute only 23 % and 2.6 % to the total weight of

the body, respectively [82]. Of course, it is mainly water that makes us heavy

(materials containing water will be discussed below).

As was considered in Chapter 8, in a thin surface layer of the complex

oxide Ag

(0.7 <x<1) at 240–340 K, there is a transition remi-

niscent of a superconducting transition [64]. Taking together, all these facts

indicate that oxides should seriously be considered as potential candidates for

a room-temperature superconductor. For example, the layers of CoO

doped

by holes will undoubtedly superconduct (see the respective subsection below).

The question is what T

value will they exhibit? Other layered oxides with the

spin S =

ground state should also be on the list of potential candidates. As a

pre-selection procedure, one must use the common requirements for materials

discussed above.

292 ROOM-TEMPERATURE SUPERCONDUCTIVITY

In addition to layered and quasi-one-dimensional oxides, one may try to

synthesize zero-dimensional oxides similar to fullerenes. An attempt to syn-

thesize inorganic fullerene-like molecules has been successful [83]. Thus, one

may in principle synthesize various fullerene-like oxides. After the doping,

some of them may superconduct.

4.2 Materials for phase coherence

A superconductor of the third group must be magnetic or, at least, have

strong magnetic correlations. While oxides can be magnetic naturally, like

the cuprates for example, organic and living-tissue-based compounds must be

doped by magnetic species which will be responsible for long-range phase

coherence.

Unfortunately, during evolution, Nature did not need to develop such mag-

netic materials. Hence, we should only rely on accumulated scientiﬁc experi-

ence and work by trial and error. The general requirements for the magnetic

properties of room-temperature superconductors were discussed above and in

Chapter 9. In addition, a few hints can be suggested.

By doping organic materials or living tissues, one should take into account

that, after the diffusion, the dopant species must not be situated too close to

the organic molecules/tissues. Otherwise, they will have a strong inﬂuence

on bisoliton wavefunctions and may even break up the bisolitons. On the

other hand, the dopant species cannot be situated too far from the organic

molecules/tissues because bisolitons must be coupled to spin ﬂuctuations.

Since in superconductors of the third group, spin ﬂuctuations must be cou-

pled to quasiparticles, the dopant atoms/molecules (at least, the majority of

them) should donate/accept electrons to/from molecules (or complex struc-

tures) responsible for electron pairing. In the framework of our project, they

must accept electrons, creating holes in a material responsible for electron pair-

ing. In all known cases, the dopant species donate/accept either 1 or 3 elec-

trons. For achieving a high T

, the dopant species should accept 2 electrons.

In this case, the electron pairs can wander around much more easily. In reality,

however, this is impractical because, after accepting/donating two electrons,

the dopant species will remain non-magnetic.

As was estimated in Chapter 9, in a room-temperature superconductor of

the third group, the value of magnetic (super)exchange energy between the ad-

jacent spins, J, should be of the order of 150–200 meV. This value is large but

realistic. In my opinion, the most difﬁcult task to be resolved is to create dy-

namic spin ﬂuctuations with ω

∼10

–10

Hz. In the cuprates for example,

a structural phase transition precedes the transition into the superconducting

state. This structural transition allows the charge stripes to ﬂuctuate quicker,

provoking a transition into the superconducting state. Therefore, synthesizing

a room-temperature superconductor, one must pay attention to its structure: the

Room-temperature superconductors 293

“distance” between failure and success can be as small as 0.01 A

◦

in the lattice

constant.

In addition to “magnetic” species which will accept electrons, one may try

to dope a material for electron pairing also by a small amount of molecular

magnets to vary the strength of magnetic correlations. Molecular magnets are

the nanoscale clusters containing a transitional metal, and they are promising

components for the design of new magnetic materials [84].

As discussed in Chapter 9, in the framework of our project, one should

start with antiferromagnetic compounds. Nevertheless, ferromagnetic materi-

als should not be excluded from the project. At present, one can ﬁnd several

publications/preprints reporting room-temperature ferromagnetism in various

compounds [85–91].

Finally, what materials to use. We are interested in materials which accept

electrons after being intercalated. For example, all the compounds listed in Ta-

ble 3.7 donate electrons. They are used to intercalate fullerenes. From Chapter

3, the materials able to accept electrons from organic molecules are the follow-

ing atoms and molecules: Cs, I, Br (atoms) and PF

,ClO

, FeCl

, Cu(NCS)

Cu[N(CN)

]Br and Cu[N(CN)

]Cl (molecules). So, one should most likely

start with these materials.

4.2.1 Artiﬁcially-induced spin excitations

From the classical standpoint, magnetic ﬁeld is detrimental to superconduc-

tivity. Experimentally, however, this is not always the case: magnetic ﬁeld can

not only destroy superconductivity but also induce it. As discussed in Chapter

2, the superconducting phase in the quasi-two-dimensional organic conductor

λ-(BETS)

FeCl

is induced by magnetic ﬁeld [26, 27]. The superconducting

phase as a function of magnetic ﬁeld has a bell-like shape, occurring between

18 and 41 Tesla with a maximum T

 4.2 K in the middle [27]. The ﬁeld is

applied parallel to the conducting layers. This experimental fact demonstrates

that what should be detrimental to superconductivity can in fact induce it. The

idea which we are going to discuss now is based on this fact.

Let us consider an idea of inducing the superconducting phase by ac electro-

magnetic ﬁeld, applicable to magnetic compounds containing bisolitons. As-

sume that we have a thin ﬁlm of an organic compound containing bisolitons,

which is doped by magnetic species. The thickness of this thin ﬁlm is of the or-

der of, or less than, a typical penetration depth in superconductors of the third

group, thus, ∼ 1000–2000 A

◦

. We know that in order to become supercon-

ducting, this complex compound should have dynamic spin ﬂuctuations with a

frequency of ω

∼ 10

–10

Hz = 1–10 THz. If such spin ﬂuctuations are

absent, one may in principle induce them artiﬁcially. Let us place the thin ﬁlm

inaweakac electromagnetic ﬁeld with ω ∼ 1–10 THz. The quasiparticles

in the ﬁlm will follow the ﬁeld, inducing spin excitations with a similar fre-